1. Field of the Invention
The present invention relates to a semiconductor device to be applied to DC to DC conversion circuits, DC to AC conversion circuits, AC to DC conversion circuits, and AC to AC conversion circuits, and in particular, to a technology to deliver estimated temperature to an external output terminal.
2. Description of the Related Art
FIG. 7 shows an example of a circuit construction of a device for DC to AC conversion or AC to DC conversion including a temperature sensing circuit and an overheat protection circuit using a conventional technology disclosed in Patent Document 1. A main circuit 1000 comprises six IGBT chips and six diode chips arranged between a high voltage positive (P) bus 400 and a negative (N) or ground bus 401. IGBTs 101 and 201 and diodes 102 and 202, which are anti-parallel connected to the IGBTs, are given the reference numerals as such; other IGBTs and diodes are given no reference numerals.
AC lines 402, 403, and 404 are connected to AC terminals that are either AC output terminals for a DC to AC conversion circuit or AC input terminals for an AC to DC conversion circuit. The gate terminal of each IGBT chip is connected to one of driver ICs 0, 10, 20, 30, 40, and 50 for driving the IGBTs. Semiconductor devices having this construction are called intelligent power modules (IPMs). Each IGBT chip is provided with a temperature sensing diode mounted thereon, and the anode and the cathode of the temperature sensing diode are connected to the driver IC. The driver ICs 10 through 50 have the same circuit constructions as that of the driver IC 0 shown in FIG. 7 and so, the constructions are not shown in the figure.
The following describes the driver IC 0. When an ON signal at a high level is given to a terminal 1, the IGBT 101 turns ON increasing the current therein and the temperature thereof. The IPM has an overheat protection circuit therein. A comparator CP in the driver IC 0 compares a forward voltage drop across the temperature sensing diode 103 with a predetermined reference voltage 11. When the voltage across the temperature sensing diode 103 decreases below the reference voltage 11, the comparator delivers an output signal at a high level. This signal blocks the ON-driving signal at a high level given through the terminal 1 at an AND gate 7. The electric potential at the terminal 2 becomes through a pre-driver circuit 8 to the ground potential to turn OFF the IGBT 101. The output signal of the comparator CP makes a switching element 12, an NMOS, turn ON to make the terminal 6 at the ground potential. During the overheat protection circuit is out of operation, the output signal of the comparator CP is at a low level, a ground potential, and so, the switching element 12 is in an OFF state and a reference voltage 15 is given to the terminal 6.
The temperature sensing diode 103 performs a characteristic of a forward voltage drop versus temperature, as disclosed in Patent Document 2, that the forward voltage drop decreases with increase in temperature, which is a negative dependence on temperature. FIG. 8 shows behavior of the signal at the terminal 6. It is shown that the voltage at the terminal 6 is the voltage of the reference voltage 15 in a suspended state of the overheat protection operation, and is the ground potential at zero volts in an operating state of the overheat protection.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. H11-142254
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2010-199490
As described above, the temperature information that is the voltage signal delivered from the terminal 6 is a binary signal that is the reference voltage 15 when the temperature of the IGBT is lower than a predetermined value and that is the ground potential when the temperature of the IGBT is higher than the predetermined value, which corresponds to the voltage across the temperature sensing diode 103 lower than the reference voltage 11. As a consequence, the conventional device cannot always watch the temperature of the IGBT in operation.